The right-angle drive came from American Science and Surplus. I don't
know how many watts it could reliably take, but I figured by the time
my work got through the triple rusty tallbike chain, the secondary
chain, and the gummy old bottom bracket that had been sitting in my
yard for years, it wouldn't be a problem.
I initially tried a bevel gear from an angle grinder, which is ideal
in a lot of ways, and gave me the opportunity to saw open an angle grinder.
Unfortunately, it was sensitive to gear placement, and my caveman
technology had too much slop. Kludges like an 8mm bearing
on a 5/16 inch shaft wouldn't cut it, never mind that universal joint.